Electronic indicator



April 22, 1952 D. L. ELAM ELECTRONIC INDICATOR Filed Nov. l, 1946 5 Sheets-Sheet l April 22, 1952 D. L. ELAM 2,594,138

ELECTRONIC INDICATOR Filed Nov. l, 1946 5 Sheets-Sheet 2 6/ lv Il 55 Z? 55 I 55 55V (56 5/ I l 35 f l l l 45 l [57.- I 1' I J jk? 5) L' will T- April 22, 1952 D. L. ELAM 2,594,138

ELECTRONIC INDICATOR Filed Nov. 1, 1946 5 Sheets-Sheet 3 s 'li D. L. ELAM ELECTRONIC INDICATOR 5 Sheets-Sheet 4 jfl yi/fa April 22, 1952 Filed Nov. l, 1946 April 22, 1952 D. ELAM 2,594,138

ELECTRONIC INDICATOR Filed Nov. l, 1946 5 Sheets-Sheet 5 Patented Apr. 22, 1952 ELECTRONIC INDICATOR David L. Elam, Roselle, Ill., assignor to Electro Products Laboratories, Inc., Chicago, Ill., a corporation of Illinois Application November 1, 1946, Serial No. '707,130

(Cl. .Z50- 27) 3 Claims. l

This invention relates to an electronic indicator, and more particularly to a system for translating a mechanical variation into electrical variation and using such electrical variation to provide a desired indication.

This application is a continuation in part of my application for a pressure indicator filed August 11, 1945, as Serial No. 610,325, now abandoned.

Rapidly changing mechanical variations vpresent difficulties and problems of indication not present in connection with slowly changing variations, and it is often desired to obtain an indication of such a rapidly changing mechanical variation. rl"he term mechanical variation as used in this specification and claims is intended to include variations pertaining to movement of a device, as for example, movement of a valve stem in a diesel injector, pressure variations in pneumatic or hydraulic mediums, and other physical variations and Wave movements with the exception of electrical variations.

For example, it may be desirable to obtain an indication of pressure conditions in the cylinder of an internal combustion engine, the cylinder of a pump, or the like, The diiculties and problems presented in obtaining such indications are readily seen to differ from the problems present in connection with obtaining an indication of slow variations. For instance, where it is desired to provide an indication of the pressure in the storage tank of a compressed air system, a conventional pressure gauge is satisfactory. However, when it is desired to measure the instantaneous pressure conditions at any time in the cylinder of an internal combustion engine which may be operating at several thousand revolutions per minute, a gauge is obviously of no use. Various types of electrical indicating arrangements have heretofore been provided for such pressure conditions, but practically all of them have operated as a function of the rate of pressure change, which is undesirable in that such devices could not measure static conditions when desired.

I have overcome these and other difficulties by providing a system comprising a source of high frequency carrier voltage Waves, means for modulating these waves as a function of instantaneous conditions in the object under test, and means for utilizing the modulated portion of such wave to indicate the mechanical variation being tested. My improved indicator opcrates on a change in reactance, and means are provided for eliminating at least a portion of the carrier wave so that modulation of such wave corresponds to the mechanical variation being measured.

A feature of my invention is that the mechanical variation is changed into an electrical variation in the form of modulation of a high frequency Wave, as, for example, a one hundred kilocycle Wave of considerable power, as 40 to 100 volts peak-to-peak swing. Providing the` electrical Variation in the form of vmodulation of a high frequency wave (amplitude modulation i in the particular case illustrated in the drawings of this application) results in an indication which is truly dependent on the mechanical variation being measured, and not on the rate of change thereof. That is, this apparatus enables static pressure or other mechanical conditions to be indicated on an oscilloscope screen, for example. whereas commercial prior art indicating arrangements are unable to do this.

envelope variation is made in such closely spaced steps, even when the time axis is considerably expanded on the oscilloscope screen, that an apparently continuous pressure curve is provided.

Another feature which contributes substantially to the desirable operation of this indicating arrangement is the particular discriminator or clipper circuit used. Ordinary rectifying arrangements fcr removing all or most of the bottom half of a modulated carrier completely remove the carrier, leaving only the variation in mechanical variation under test, a corresponding manually adjustable impedance means, and two other impedance elements in a single pickup 'unit in a bridge arrangement. Use of a bridge arrangement and location of all the ele' y ments thereof in the unit mounted adjacent the source of mechanical variation, as for example on a wall of a chamber having therein a variable pressure to be indicated, provides means which can be satisfactorily balanced and which will provide a correct electrical indication of mechanical variations in the chamber. In this embodimentnthe provision of the manually adjustable balancing means is a very important factor, particularly Where the pick-up is used in association with a chamber at a relatively high temperature as for example a cylinder of an internal combustion engine. N o amount of balancing the bridge circuit on a laboratory bench would mean anything when the temperature of the pick-up was considerably raised in use, probably several hundred degrees above that at the working bench. The convenient manual adjusting means, however, enables the pick-up to be brought up to operating temperature and then properly and accurately balanced with a turn Moreover.. by making it possible to use high frequency the Or' two of a wrench, so that the, bridge circuit is properly balanced at the operating temperature, and the electrical indication is brought to zero when the pressure in the chamber is at zero, with variation of the pressure varied impedance element then providing known indicating variations.

Another embodiment of my invention utilizes a variable reactance in series with a fixed impedance to translate the mechanical variations into electrical variations, and then utilizes electronic means to eliminate at least a portion of the carrier wave, such electronic means oomprising overbiased tubes, the bias on at least one of said tubes being variable in order to determine the amount of the carrier wave which is to beA eliminated. In this embodiment of my invention means are provided to overcome the plate to cathode capacitance of the aforementioned tubes in order that the entire unmodulated portion ofthe carrier wave may be eliminated if desired, together with means for selectively removing the carrier component of the modulated voltage wave while retaining substantially unaltered the outline of the modulated envelope.

Iny this embodiment of my inventionA means are also provided for changing the impedance of the output circuit to a desired value in order that the wave may be fed to the deflection plates of an oscilloscope;

These and other features and advantages of this invention will be apparent' from the following specification and the drawings. in which:

Figure 1 is a longitudinal sectional view through a pick-up embodying my invention; Figure 2 is a longitudinal sectional view of the output transformer, along the line 2--2 of Figure 3; Figure 3 is an endk view of the transformer shown in Figure 2; Figure 4 is a schematic circuit diagram of the pick-up arrangement shown in Figure 1; Figure 5 is a schematic circuit diagram of the entire system, including the pick-up and the oscillator, amplier, discriminator, and power supply associated therewith; Figure 6 is a longitudinal sectional View of another form of pick-up comprising a modified form of my arrangement embodying certain of my invention; Figure 7 is a fragmentary sectional view of one of the reactance elements; Figure 8 is a circuit diagram of the pick-up arrangement shown in Figure 6; Figure 9 is a longitudinal sectional view through a .pickup comprising another embodiment of my invention; Figure 10 is an enlarged fragmentary longitudinal sectional view through the bottom f portion of a pick-up comprising another modication of my invention; and Figure 11 is a schematic diagram of a circuit designed to be utilized with the form of my invention shown in Figures 9 and 10.

In order to make the description of the entire system or combination more readily understandable, I will rst describe one form of pick-up uniti which operates to modulate the high frequency waves as a function of mechanical variations.

While I may employ either an inductive or capacitive form of pick-up, I prefer the capacitive form because it permits high frequencies of the order of 100 kilocycles to be used. The inductive form of pick-up will perform satisfactorily with carrier frequencies in the neighborhood of several thousand cycles, and this is fully satisfactory where a relatively slow speed variation is being measured, particularly where there are no substantially instantaneous pressure changes in the general pressure change curve. However, where it is desired satisfactorily to indicate pressure changes which are recurring at a relatively high rate of speed, as in an engine operating at four or five thousand revolutions per minute, or where there are brief but highvpeak transient surges in the general pressure curve it is necessary to use a high frequency carrier for satisfactory results. Under such conditions as those last mentioned a carrier frequency of at least 10,000 cycles, and preferably of at least 50 or 100 kilocycles, is necessary. At these frequencies the inductive pick-up or modulating portionrof my system is not fully satisfactory, and a capacitive pick-up is preferable.

Accordingly, the capacitive type of pick-up unit has been illustrated first, and this will now be described, reference being had more particularly to Figures 1-4. In the particular embodiment of my invention illustrated in these figures, the pick-up comprises a generally cylindrical outer housing Iii made in several parts for convenience. These are here shown as comprising a central barrel portion Ia, an upperl portion Ib, a lower portion Ic, and an end member Id. The end member Id has a reduced threaded portion Iiid' adapted to be received in a correspondingly threaded opening in a wall I I of the chamber where the pressure indication.

is to be made, as a wall of the cylinder of an internal combustion engine or a pump, for example. 'Ihis end member Iid has passageway means therethrough providing communication between the interior of the pressure chamber and the interior of the pick-up housing, this being here shown as the single bore or opening IIJd". All metal portions of the housing and of the elements within it, and particularly the reactive elements, are preferably made of the same metal, and preferably of a metal with a relatively low temperature coefficient, as for example steel.

Mounted in the pick-up housing II) are a pair of impedance elements having capacitive reactance, these being here indicated in general as I2 and I3. The capacitive reactance element I2 is here shown as comprising the xedly mounted plate I2a and the movable cooperating plate I2b, air acting as a dielectric therebetween. The plate IZb is preferably a thin diaphragm of tough metal, as spring steel or phosphor bronze, and this diaphragm flexes as a function of pressure changes in the chamber eiective against the lower side of the diaphragm (speaking with respect to the particular position of the parts as illustrated in Figure l) this exing or movement of the diaphragm varying the distance between the condenser plates I 2a and IZb and thus varying the capacity of this impedance element I 2. In practical pick-ups .which have been constructed in accordance with my inventions, it is desirabley to keep the housing diameter down to about 11/2 inches or less, and the capacity present in the impedance element I2 (when the diaphragm is unstressed) is of the order of 10-15 microfarads, this capacity being increased to as much as double or more its initial value when the diaphragm is exposed to high pressure. The diaphragm plate I2b is fixedly mounted at its edges in the housing portion Iiic, as illustrated; and the plate member Iza is fixedly mounted by insulating mounting members here identified as I2c and I2d. The edges of the diaphragm plate I2b are in direct contact with the housing, as this plate and the housing are grounded in operation, as by the connection between the housing end member and the wall of the pressure chamber.

The other capacitive reactance element i3 is here shown as comprising a iixedly mounted f plate I3a held in place and suitably insulated by insulating supporting members isc and I3d. Cooperating with this iixed plate member Ita is another condenser plate member I3b, this comprising the face of a threaded adjustment member having means enabling convenient rotation thereof at one end, as a hexagonal portion here identified as I3b, this adjustment member being threaded into the upper housing member Iiib.

A lock nut I4 is associated with this member to l enable it to be locked in adjusted position in the particular form shown, although it will be understood that any desired locking arrangement may be used.

The pick-up housing also encloses another pair of corresponding impedance elements here shown as comprising the resistors identified as I5 and I6. In the particular unit being described these may have a resistance value of the order of 100,000 ohms for satisfactory results, although it will be understood that resistors which vary substantially from this value will still provide operative results. Increasing the resistance value gives some increase in sensitivity, but maximum power transfer is effected when the impedance of this pair of elements is substantially the same as the impedance of the capacitive elements at the particular frequency being used, 100,000 cycles in the particular device being described. The pick-up housing also ncorporates a transformer here identified in general as Il, this transformer being shown more particularly in Figures 2 and 3.

Referring now to these last mentioned figures, the transformer will be seen to comprise a central core portion Ila, as of laminated or powdered metal of high magnetic permeability, a secondary or output winding I'Ib, and a split or balanced primary winding comprising the parts Ilc' and I'Ic, an electrostatic shield comprising a grounded split metal sleeve IId being interposed between the coils in order to ensure magnetic transfer and to prevent capacitive reflection or reaction on the circuit.

Referring now more particularly to Figure 4, the manner in which the elements within the pick-up housing are connected will be described. The circuit will be seen to comprise a bridge consisting of the corresponding capacitive reactance elements I2 and I3 on the one hand and the corresponding resistance elements l5 and I6 on the other hand. The capacitive element I2 is indicated in the diagram by a symbo1 showing it to be variable, as the capacity of this element varies during operation as a function of the variation in the pressure conditions effective upon the diaphragm. While the capacitive element I3 is represented by a symbol indicating its capacity to be iixed, it will be understood that this is fixed only in operation; i. e., as has been explained before, the capacity of this element is manually adjustable to balance the bridge after the pick-up unit has been brought up to operating temperature, and it is only thereafter that it remains fixed. The resistors I5 and I6 are, of course, always fixed. The high frequency voltage or carrier waves are supplied, through means to be hereafter more fully described, be-

4tween a pair of opposite corners of the diamond bridge connection, these being vhere identified as the corners I8 and I9; and the desired modu lated high frequency output is taken oil from the other pair of corners of the diamond bridge arrangement, these being here identied as 20 and 2I. The actual input to the device, while here indicated as a two-wire input for simplicity of illustration, is actually a one-wire input, as by a shielded cable, to the terminal 22, since the other part of the circuit is completed through ground, the corner I9 of the bridge arrangement being grounded, this connection point I9 comprising the housing of the pick-up unit in practice. The amplitude modulated high frequency output, modulated as a function of variation of the capacity of the condenser element I2, is developed between the points 20 and 2| through the primary I'Ic of the transformer, this being shown as a single winding for simplicity of illustration, and is transferred to the secondary I'Ib. The secondary has one end grounded to the casing or housing of the pickup unit, and the other end connected to the termnal 23, providing the output terminal of the pick-up unit adapted to be connected to the ampliiier and discriminator. It will be readily apparent that if a high frequency wave is developed between the terminal 22 and ground by any appropriate means, as an oscillator, that there would be no output developed between the output terminal 23 and ground so long as the bridge is balanced. Accordingly, after the pick-up is brought up to operating temperature, where such is different from normal room temperature, the capacity element I3 is manually adjusted to bring the bridge to balance when pressure in the pressure chamber is at zero. Thereafter increases in pressure in the chamber, during operation of the indicating system, vary the capacity l2 and thus vary the amount of high frequency carrier developed between the output terminal 23 and ground; i. e., provide an output which consists of a carrier wave amplitude modulated as a function of the pressure variations, in the particular case here being described, although it will be apparent that other forms of modulation or bridge variation could be used.

Referring now more particularly to Figure 5, the general system and its operation will be described. The system illustrated comprises a pick-up unit A, as for example of the character just described; an oscillator B for supplying the high frequency input to the pick-up; an amplifier C for amplifying the modulated output of the pick-up; a discriminator D for providingra single curve indication on an oscilloscope screen. Without loss of the high frequency waves; and a power supply unit E. The output of the discriminator is supplied from the terminal 24 to the desired indicating means, as the vertical deflection plate system of an oscilloscope, this latter not being shown since yany suitable commercial oscilloscope may be used.

Referring first to the power supply unit E, this is here shown as comprising the transformer 25 having a primary 25a adapted to be connected to a suitable source of electrical energy, as the conventional volt, 60 cycle alternating current. The transformer includes a high voltage center grounded secondary 25h having its ends connected to the two plates of a full wave rectier tube 26, which may be of tube type No. 5Y3GT, for example. A low voltage winding 25oA supplies cathode current for the rectifying tube; and another low voltage secondary 25d supplies cathode-filament current for thetubes frequency by-pass condenser 'plate of a voltage regulating tube 29, which may beof tube No. VR150, this tube having its other terminal or cathode grounded. The desired lter `'effect is obtained by associating with it elements 2l and 28 and a pair of condensers 30 and v3| of vsuitable value, as for example 10 microvfarads. A regulated source of plate or B 'plus voltage maintained substantially constant at 150 volts is thus provided in the lead or wire 32.

Referring now more particularly to the oscillator, that portion of the circuit diagram surrounded by the dotted line box 33 indicative of grounded shielding, this will be seen to comprise as' its principal operative element the pentode tube 34, which may be of tube type No. BVS. This tube has its positive .plate supply delivered to its plate and screen grid through further l tering means here shown as comprising a choke 35, which may be of 50 millihenries inductance, a condenser 36, which may be oi niicroarad capacity, and a resistor 37, which may be oi 4,000 ohms. The plate and screen grid are oy-passed to ground through a condenser 30, which may Aalso be of .25 microfarad capacity. The oscillator circuit is of a conventional type using a tapped inductance 30 shunted by a fixed capacity 40, which may be of .0005 microfarad capacit in conjunction with an inductance of five milli henries, to vprovide an operative osciilator frequency of about 100 lrilccycles. One end of the tank circuit is connected to ground; the other end of the tank circuit is connected to the signal grid of tube 34 through a grid leak and condenser combination which may comprise a resistor di vof.15,000 ohms and a condenser 02 of .0015 microfarad; and the tap oi of the inductance 30 is vconnected to the cathode and suppressor grid.

Heater current is supplied to the heater .of the tube 34 through a circuit including the wire $3 on the one hand, and the Wire 4d cn the other hand, the latter passing through another iilter section here shown as comprising a choke d5, which may be of 1.5 millihenries and a condenser 46 which may have a capacity of .25 microfarad;

and the heater terminals are shunted by an audio -which have a capacity of .25 microfarad. This osciliator is capable of providing high frequency voltage waves of considerablepower, of the order or y100 i volts peak-to-peak swing across the output terfminals; `and these high frequency waves are dclivered through theblocking resistor 53, which may have a value of 10,000 ohms, to the input 'terminal 22 of the pick-up.

The high frequency or carrier wave thus delivered to the pick-up is modulated by the bridge circuit therein as a function of the pressure or other mechanical variation being indicated, 'as

vhas'been heretofore more fully described, and the 5! and filter condenser 5in, which might have a value of 10 microfarads, and the two .1 megohm resistors 52 and 53. The tubes are here shown as self-biased by the cathode resistors 54 and 55 each shunted by the by-pass condenscrs 56 and 5l'. For the particular tubes named the cathode resistors may have a value of 5,000 ohms and the bypass condensers capacities of .015 microfarad. Grid loading resistors 5i? and 50 are used, and the resistor 53 may have a value of .5 megohm and the resistor 59, of the variable type, a maximum resistance of the same yalue. Coupling be tween the tubes is here shown as eiiected through a condenser 60 which may have a capacity of .0001 microfarad.

The amplitude output of the second stage of this amplifier, developed across the plate load resistor 53, is here shown as applied through the coupling condenser Ei,'which may have a value of .005 microfarad, to the input of the discriminator section comprising as its principal element the tube 52, which may be of tube type No. 6SF5. Biasing voltage for the cathode of this tube is developed by means or" a bleeder Vresistor connected between the positive plate supply and ground and comprising a iixed resistance element section 53, which may be of 75,000 ohms, and a variable resistance section 511, which may be of 5,000 ohms. The slider or potentiometer contact arm B5 is connected to the lower end or a cathode resistor 65, which may have a value of 10,000 ohms; and the biasing portion of the resistor 55 operative at any particular time is by-passed to ground by a condenser 5l, which may have capacity of .5 microfarad.

This discriminator or clipper circuitarrangement is of considerable importance and advantage in an indicator system of the kind here being disclosed. The output oi the system is taken off from the terminal 24 connected to the upper end of the resistor 66 through a coupling condenser S8, which may have a capacity of .05 vrnicrofarad, and resistor 56 is un-by-passed, i. e., it is not bypassed to ground with a condenser in the conventional manner. By by-passing the operative portion of the bias-determining resistor 64 by the condenser 6l, the lower end of the resistor 65 is always kept at ground potential from an A. C. standpoint; but the upper end of this resistor is at a voltage above ground determined by space current flow in the tube 52, not only with respect to the amplitude variations corresponding to the pressure, but also with respect to the high frequency voltage variations of the positive portions of the carrier wave. Adjustment oi the amount of bias on the cathode of the tube 5,2, by variation of the position of the contact .55 on the resistor 50, determines the amount of the negative swings of the carrier voltage which are chopped off or removed, and preferably this is vso adjusted that practically all of the negative portions of the carrier voltage swings are eliminated from the output delivered to the terminal 25 and thus to the oscilloscope. However, the positive swings of the high frequency wave go through and provide a resultant indication on the screen of the oscilloscope tube which comprises eiectively a completely shaded in area between the zero pressure or horizontal axis and a top curve representing the instantaneous pressure. This top curve is very smooth because of the high frequency of the carrier wave which may lbe used with this system, even when the time .axis is stretched out considerably by adjustment of the oscilloscope controls. .Moreoven because of the relatively very high frequency of the carrier wave impulses, very brief transient pressure variations over a time interval as short as of the order of a thousandth of a second areclearly shown up on the oscilloscope screen. Moreover, as has been mentioned before, where this indicating system is being used for devices which can be stopped at a certain pressure condition, as a pump or the like, the pressure is indicated at a particular level, or with a very small diminution as it leaks oi through the Valves and around the piston packing, by virtue of the fact that the carrier Waves or positive voltage swings of the high frequency waves provide an indication at the appropriate level above the zero axis on the oscilloscope tube screen without any change in pressure, since the bridge circuit used will deliver to the output waves of an amplitude determined by the capacity of the capacity element |2, regardless of whether this capacity is varying or not.

Another form of pick-up is shown in Figures 6 and 7, with its circuit diagram being shown in Figure 8, this pick-up again making use of the bridge principle with all four impedance elements in the pick-up housing, but the two variable impedance elements in this case being inductance elements. This form or modification of my pickup will now be described with particular reference to these last-mentioned figures, and in order to shorten the description reference numerals 10 higher than those used in Figures 1 and 4 will be associated with parts corresponding in their function.

In this latter form of my invention there is again a general cylindrical housing here identified as 80, adapted to be threadedinto an appropriate opening in the wall 8| of a chamber wherein the pressure variations are to be indicated. Here the reactance elements identified in general as 82 and 83 are inductive rather than capacitive in character. The element 82 comprises a core piece 82a cooperating with a ilexible diaphragm 82h. In this case the core piece 82a is encircled by an inductive winding 82j, the inductance of this winding being varied by variation of the gap between the center of the diaphragm 82h and the adjacent face of the core piece 82a, the magnetic circuit being completed through permeable material in the mounting in the upper end of the core piece and in the diaphragm 82h, this latter being necessarily of magnetically permeable material in this form of the invention. The corresponding reactance element 83 has its core piece 83a Within a winding 83j, the inductance of this winding being varied by movement of the face 83h of the adjustment member toward and away from the adjacent core piece face. This again enables the bridge circuit (incorporating inductive reactance elements in this case instead of capacitive reactance elements) to be readily and conveniently balanced by manual adjustment with the pick-up under operating conditions.

The pick-up also includes other parts and elements necessary to render it operative, including the resistors 85 and 00 and the transformer 8l. As may be best seen in Figure 8, these are connected in a diamond shaped bridge arrangement with the carrier frequency developed between the corners 88 and 00 of the bridge and the output developed across the corners 90 and 0| of the bridge delivered through the primary of the transformer 8l, the secondary of this transformer being connected to ground and to the output terminal 93 of the pick-up. This form of pick-up is fully satisfactory at carrier wave equencles at the order of 2,000 or 3,000 cycles per second, and thus provides satisfactory indications under conditions where the time axis on the oscilloscope screen is not to be unduly stretched out, and where no exceptionally brief time duration transient pressure variations are encountered.

A modifled form of a pick-up unit is shown in Figure 9, this modiiication doing away with the balanced transformer and the bridge circuit of the first described embodiment of my invention and utilizing electronic means to eliminate the undesired portion of the carrier wave. The

pick-up unit is housed in the generally cylindri-` cal housing |00 which for convenience is made up of several parts. An upper central portion i00a has an interiorly threaded upper end into which is iitted a complementary threaded upper end portion |00b, and said upper central portion has an exteriorly threaded lower end over which is fitted the complementary threaded lower central portion l00c. An end portion |00d is threaded into the lower end of the portion |00c, said end portion having an exteriorly threaded portion |0011 adapted to be receivedin a correspondingly threaded opening in communication with the mechanical variation to be measured, as a wall of the cylinder of an internal combustion engine or a pump, for example. This end member |00d has passageway means therethrough providing for communication between the interior of the chamber having the pressure Variation or the other mechanical variation to be measured and the interior of the pick-up housing, this being here shown as the single bore or opening |0041". A compression lock nut |00e is threaded onto the upper portion of the lower central member in order that plates of the reactance element to be hereafter described may be spaced as desired. An outer cover |00f surrounds the upper centr-al portion as shown.

The upper portion of the end member |001) is recessed as shown at |00b, and an input lead |0| and an output lead |02 are mounted within said recess, said leads being held in position by the insulating member |03.

The upper central member |00a carries a transformer between the insulating mountings |04 and |05, said mountings being securely fastened in place by means of the screws |06. The transformer comprises a core |01 of permeable material, a secondary winding |08 and a primary winding |09. The secondary winding may comprise about 500 turns of fine insulated wire wound next to the core, and the primary may be made of approximately 2,000 turns of such wire wound directly over the secondary. 'Ihe core may comprise about eight pieces of black iron wire each of .034 inch in diameter or a powdered iron core.

The input lead |0| extends through the upper transformer mounting and is secured by soldering or other suitable means to one end of the primary winding, while the output lead |02 extends through the transformer mounting and is secured to one end oi' the secondary winding. The other end of the secondary winding is grounded by means oi' the soldering lug 0 which is held in contact with the housing |00 by means of the screw |06, while the other end of the primary winding is secured by soldering or other suitable means to the soldering lug and said soldering lug is connected by means of the nut ||2 to the condenser plate stud I3.

Said condenser plate stud is carried by the accorse upper center portion lima, ofthe housing, being centered therein by means or the insulating mounting plugs H' and H5, and at the bottom of said condenserV plate stud is the iianged portion lie, comprising one plate of a reactance means `here shown as a condenser I I'i. The other plate of said condenser is the diaphragm H S which is carried by the end portion id of, thehousing and is spaced therefrom by the spacer gasket H9. The diaphragm H3 is preferably a thindiaphragm of tough metal as spring steel or Phosphor bronze in order that it may ex as a function of pressure changes in the chamber eiective against the lower side of the diaphragm, thereby varying the distance betweenthe condenser plates H and H3 and thus varying the capacitive reactance of this condenser. The diaphragm H3 is `fixedly mounted at the edges as shown inv Figure 9, and the condenser plate stud ||3 which carries the plate member 4| I6 is iixedly mounted by means of the insulating plugs IM and H5. The edges of the diaphragm H8 are in direct contact with the housing, as this plate of the condenser and the housing are grounded in operation through the connection between the housing and thev wall of the pressure chamber.

The plates of the condenser Ymay be sep-A arated any desired amount by adjusting the member loller (which carries the plate |56) with respect to the member |030 (which carries the plate I8), and the said plates may be locked at .any desired separation by means of the compression lock nut Hille.

VIn operation of this embodiment of pickup device a high frequency carrier voltage wave is fed into the pick-up unit through the input tern minal lill. Since the primary |59 of the transformer and the condenser just described are in series, a voltage drop will appear across the circuit including theses elements, and such voltage will be distributed between the primary of the transformer and the `condenser depending upon the inductive reactance of the transformer primary and the capacitive reactance of the condenserV at the particular frequency employed. It is desirable that the series circuit comprising the inductive member and the capacitive member is not resonant at the frequency employed, and in one embodiment of my invention which I have constructed I have obtained satisfactory results by using a transformer primary winding with an inductance of 25 millihenries. In order to resonate at 100 kc. (which is the frequency used for the carrier wave) the condenser Hl would have to be of 100 micro-rnicrofarads ca-V pacitance, while in actual practice this condenser has a capacitance varying from 5 to 10 micromicrofarads.

As the condenser plate ||8 nexos as a function of the mechanical variations to be indicated (in this case varying pressure in a chamber) the capacitance and hence the capacitive reactance of the condenser varies, thus changing the distribution of the total voltage of the carrier Wave across the respective inductive and capacitive members of the circuit. Obviously these changes occasioned by the varying capacitive reactance could be utilized if taken oi either across the condenser or across the Ainductive member, and I show such electrical variations as being taken oi from the secondary of the transformer which is connected between the output terminal |82 and ground. Thus the carrier wave is modulated according to the mechanical variations acting Vupon the diaphragm i3, and such modu lated carrier wave is taken on by means of the secondary winding Hi8 which is inductively coupled to the primary winding it, and between the output terminal |82 and ground there exists an alternating carrier voltage which is modu-v lated in accordance with the mechanical variations to which the diaphragm H8 is subjected.

Since the electronic apparatus which may be used with my pick-'up unit is always separate from the pick-up unit and may be located a considerable distanee from the pick-up, a transmission lineis used to carry the impulses from the output terminal of the pick-up unit to the input oi the electronic apparatus. I have found it preferable to utilize a step-down transformer `,in the pick-up unit,r thus making it possible to use a low ima pedance transmission line, as, for example, a 15G ohm line. By this means it is possible to use the pick-up at a considerable distance from the electronic apparatus with only negligible loss, and at the same time pick up only a minimum of undesired noise or other signals. Means may be provided at the input to the electronic apparatus to step the voltage from the transmission line up to a desired level, as, for example, the step-up transformer shown at 25| in Figure 1l.

InoFigure iOra niodied foi-mor my pick-up unit is shown, only the bottom portion of such unit being shown in the drawing since the upper portion is the same as. theeppere-tus showin Figur@ 9. In Figure 10 for convenience of description corresponding parts of the apparatus will bc esignatedA by numerals higher than those in Figure 9. The apparatus of .Figure 10 is pern ticulerly .designed t-o pick-up variations 0f. movement, as for example the movement of a valve stem in a diesel injector, rather than to picku up variations in pressure as was the apparatus heretofore disclosed.

In the drawing the housing Zi again consists of various parts, the lower central portion only beings-hewn as Zeile. Such portion is illustrated as having an interiorly threaded base portion adapted to support the pick-up on the apparatus under test, as the injector head 225. The reactance member 2H comprises the fixed element Elie which forms one plate of the condenser and isflxedly mounted on the condenser plate stud 2 i 3 which is supported in the insulating block 2|5 as in the last described modification of my invention. The other plate of the condenser is the cupshaped electrode li8 which is mounted on injector valve stem 225 and is secured against movement relative to said valve stem by means o nut 227. As the valve stern 2265 moves up and down the capacitance between the plates 2 6 and 2|S will change, and consequently the capacitive reactance of the condenser 2|? formed by said plates will also change.

The condenser is connected in series with the primary of a transformer, as in the modification oi my invention disclosed in Figure 9, and such series circuit is adapted to be energized by means of .a carrier voltage Wave in the same manner as heretofore described, while the varying potential occasioned by the varying capacitive react-- ance of the, condenser is taken off from said piel:- up unit by means of the inductively coupled secondary of the transformer in the manner heretofore described.V

The system is illustrated as comprising "a pickup unit A, as for example a unit of the character sho-wn in Figure 9 or Figure l0; an oscillator B for supplying the high frequency to the input pick-up; electronic means for removing substantially all of the high frequency carrier wave indicated at C; an impedance changing device D for changing the impedance of the electrical variation to a desired value so that it may be applied to an oscilloscope; and a power supply unit E. Obviously if desired the output waves from the pick-up unit A may be amplified by means of a proper circuit, as the circuit shown at C of Fisure 5.

Referring first to the power supply unit E, a transformer 230 has a primary 230e adapted to be connected to a suitable source of electrical energy, as a conventional 110 volt 60 cycle alternating current line. A fuse 23| and a switch 232 are shown in said line for obvious purposes of control. The transformer 230 includes a high voltage secondary 235D having its ends connected to the two plates of a full wave rectifier tube 233, which may be of tube type No. Y3GT, for example. A low voltage winding 230e supplies cathode current for the rectifying tube; and another low voltage secondary 230d supplies filament current for the tubes for the other electronic portions of the apparatus, such filament connections not being shown in the drawing since they are conventional.

The positive rectied output of the rectier tube 233v is here sho-wn as being developed through a filter arrangement comprising the choke coils 234 and 235, which may each have a value of l0 henries and the resistor 235 which may have a value of 1250 ohms, to the plate of a voltage regulating tube 231, which may be of tube type No. VR150, this tube having its other terminal or cathode grounded. The center of the secondary 230D of the transformer is connected to the cathode of another voltage regulating tube 238, which may be of tube type No. VR75, and the plate of such tube is grounded. The desired filter effect is obtained by means of the choke coils and resistor in connection with the condensers 239, 240 and 24| which are connected between the two leads just described, and which may each have a value of ten microfarads, and by means of the Y condensers 242 and 243 which are connected respectively between each of the aforementioned leads and ground. rIhis apparatusprovides a regulated source of plate or B plus voltage in the lead 244, such voltage being maintained substantially constant at 150 volts above ground, and a source of bias voltage in the lead 245, this voltage being maintained substantially constant at volts below ground.

Referring now more particularly to the oscillator, which comprises that part of the circuit diagram surrounded by the dotted line box 24.5 indicative of grounded shielding, the pentode tube 241, which may be of tube type No. 6V6 comprises the principal component. This tube has its positive plate supply voltage delivered to its plate and screen grids through 'further filtering means here shown as comprising a choke 248, which may be of millihenries inductance, a condenser 249 which may be of .25 microfarad capacity, and a resistor 250 which may be of 4,000 ohms resistance. The plate and screen grid are by-passed to ground through a condenser 25|, which may also be of .25 microfarad capacity. The filter circuit comprising the choke 248 and the conmay be applied to an oscilloscope.

14 denser 249 are also shielded as indicated by the dotted line 252.

The oscillator circuit is of a conventional type using a tapped inductance 253 shunted by a iixed capacity 254 which may be of .0005 microfaradl capacity, in conjunction with an inductance of 5 millihenries, to provide an oscillator frequency of about kilocycles. One end of the tank circuit is connected to the signal grid of tube 241 through a grid leak and condenser network which may comprise a resistor 255 of 15,000 ohms and a condenser 256 of .0015 mcrofarad. The tap off of the inductance 253 is connected to the cathode and suppressor grid. Heater current for the lament of the tube is supplied from the secondary 23M, a condenser 251 which may be 0f `-.25 microfarad capacity being shunted across said lament. This oscillator is capable of providing high frequency carrier voltage waves of considerable power of the order of 100 volts peak-to-peak swing across the output terminal; and this high frequency carrier wave is delivered through the blocking resistor 253 which may have a value of 10,000 ohms to the input terminal |0| of the pick-up.

The high frequency or carrier wave delivered to the pick-up is modulated therein as a function of the mechanical variation under test, as has been heretofore more fully described, and the function of that portion of the circuit just described is similar to the function of the corresponding portion of the circuit in Figure 5 to wit to deliver a high frequency carrier wave of substantially constant amplitude to the pick-up unit. However the type of vpick-up unit illustrated in Figures 9 and 10 does not include means for eliminating the unmodulated portion of the carrier waves as do the pick-up units illustrated in the Figures 1 and 6, and the balance of the circuit to be hereafter described provides means for electronically eliminating substantially all of the unmodulated portion of the carrier Wave and for changing the impedance of the modulated envelope to a desired impedance so that the wave Means are also provided for removing that portion of the carrier within the modulation envelope if desired. and switching means are provided for selectively connecting either the modulated envelope without the carrier or the modulated envelope including the carrier to the impedance changing device. In this connection I have designed apparatus to provide that the modulated envelope will have the same amplitude when the switch is in either position.

While the means for eliminating the unmodulated portion of the carrier wave might comprise one or more triodes 01 other tubes biased below cutoff, I illustrate such means as comprising the double diode rectifying tube 250 which may be of tube type No. GHG. It will be understood that two separate diodes may be used in place of the double diode if desired. The modulated output from the pick-up unit is inductively coupled to the rectifying circuit by means of the transformer 25| having a primary 25m and a secondary 26H7. The primary is connected between the output terminal |02 of the pick-up unit and ground, and the carrier wave which varies as a. function of the mechanical variation being measured is inductively coupled into the secondary 26|b and fed to the rst plate (reading from the left) in the rectifying tube.

As is commonly known in the art, if the rst cathode of the rectifying tube 260 were at ground netentialA and the first plate of saidtubewere at the instantaneous potential of the modulated carrier, the tube would conduct whenever such carrier swings positive, and the output of the tube would include the positive half or" the unmodulatedY carrier as well as the positive modulated @nation thereof. In order to avoid this and to eliminate the unmodulated portion of the carrier I overbias the first plate of this reotifying tube,y and provide a bias voltage therefor which is substantially equal to one-half the peak-to"-V peak amplitude of the unmodulated portion of the carrier wavey which is delivered to the input of Saidtu-be.

This bias voltage is delivered to the tube through thel lead 2% which runs between the cathode of the voltage regulating tube 23S and ground and incorporates therein the resistor 252 whichmay be of the value of 2,000 ohms, the potentiometer 253 which may have a value of 5,000

ohms and the resistor 261|` which may have a value or 500 ohms. Since the voltage at the tube 238 is '75 volts below ground, there is normally a D. C. voltage which varies from minus 55 volts to minus 5 volts along the potentiometer 263. The slider 25311 of such potentiometer is connected to the bottom end of the secondary 26 lb, and consequently without reference to the voltage of the carrier wave there is maintained on the first plate of the rectifying tube 2GB a bias voltage of anywhere from minus 5 to minus 55 volts, depending upon the position of the slider 253s. In operation of my apparatus the slider is adjusted until the bias voltage is equal to one-half of the peak- .to-peak amplitude of the unmodulated portion of the carrier wave, and in consequence thereof the rectiiying tube- .'llll only conducts during the modulated portion of said wave.

T have found that this portion of the circuit fails to remove the entire unmodulated portion of the. carrier wave because of the interelectrode capacitance between the iirst plate and the rst cathode of the rectifying tube, and l' overcome the etect of this interelectrode capacitance by using two diodes in cascade rectication. In the drawing both diodes are shown as beingr part of the same tube, although it will be understood that if desired, separate single diodes may be used instead of a tubo type No. 6H6 or other dual diode` tube.

The rectified carrier with-a part of the un` modulated portion eliminated therefrom is Vtaken offrom the -top of; the cathode resistor 265 which may have a value of 50,000 ohms, and is fed to the second plate of the tube through the automatically biasing grid leak network comprising a parallel combination of the resistor 250 which may have a value of 50,000 ohms and the condenser 2,61 which may have a value of .0005 microiarad, The lead including suchA network isV bypassed to ground by the small condenser 205 which may have a value of .0001 microfarad.

. The grid leak network automatically provides a bias on the second plate of the rectifying tube so that such tube only conducts when the voltage exceeds the remaining portion of the unmodulated carrier voltage, and consequently only the positively modulated portion of such carrier is passedjthrough said tube, thereby providing an elective electronic means f oreliminating substantially all of the unmodulated portion of the carrier.

In the output circuit of the tube 250 l. provide a resistance-capacity network together with switching meansV movable between two positions,

this arrangement being designed to deliver to the following impedancechanging stage either the modulated envelope including the carrier wave, 0r to remove said carrier wave from themodulated envelope and deliver the integrated envelope alone.

The resistance capacity network comprises the condenser 259 which may have a value of .000,1 microfarad and the parallel cathode resistance including the resistor 2id which may have a value of 5,000 ohms and the potentiometer 21| which may have a value of l megohm. The ganged switches 212 and 273V are operabler to selectively deliver to the impedance changing stage D either the modulation envelope and carrier wave complete or merely the integrated.mod-V ulation envelope. 1t will bev noted that One end of each of the resistors 210 and .21| is connected to the second cathode of tube 250, and that the other end of the resistor Mil is connected to one terminal of the switch 212, while theother termi,- nal of the potentiometer 21| is connected to ground. With the switch in the position as shown Y in the drawing the small resistor 210 is connected output being delivered to the jack 28|.

in parallel with the large potentiometer 21|` between the second cathode and ground, and Said resistor 210 eiectively shorts out the potentiometer 21| and lowers the impedance between said second cathode and ground, thereby practically nullfying the by-passing eiect of the condenser 269 since said condenser has a very high capacitive reactance at the carrier frequency used, namely kilocycles. With the switch in this position only a very small portion, if any, of the carrier wave contained within the modulation envelope will be by-passed to ground, and the modulation envelope and carrier substantially complete therein will be delivered to the impedance changing stage D through theV lead 214 connecting the top of the resistors to the switch 213, through said switch, and through the lead 215.

However should the ganged switches 212 and 213 be thrown to the position other than that shown in the drawing the resistor 210 would be disconnected from ground and thereby removed from the circuit, and the capacitive reactance of the condenser 259 will now be in parallel with the one megohm potentiometer 21|. In this event the high carrier frequency will be by-passed through the condenser 23S to ground, While the lower frequency modulation variations will develop a voltage across said potentiometer, and this integrated voltage, being the modulation envelope with the carrier removed therefrom, will be delivered to the impedance changing stage through the lead Elt which connects the slider 21m to the switch 213, through said switch, and through the lead 215. The slider 21m enables the circuit to be adjusted so that a modulation envelope of the same amplitude will be delivered to the frequency changing stage when the switches 212 and 213 are in either position.

The impedance changing stage D comprises the tube 211 which may beY of tube type` No. 6.15. Plate voltage for said tube is delivered from the positive high voltage source through the lead 2G14 and the plate dropping resistor 210 which may have a value of 20,000 ohms. Bias for said tube is developed across the resistor 254 which is bypassed by the condenser 219 which may have a value of .5 microfarad, and the output is taken from the cathode of said tube across the resistor 280, which may have a value of 2,000 ohms, said One asegiee l? terminal of said jack is grounded as shown, and the jack is adapted to be connected to a conventional oscilloscope.

This last described circuit comprises a cathode follower which changes the impedance of the modulation envelope to a desired figure, as 2,090 ohms in the case illustrated. Furthermore the circuit acts as a discriminator in the same manner as the circuit illustrated in Figure 5, for the cathode resistor 288 is un-by-passed while the bias determining resistor i6-fl is bypassed, thereby always maintaining the bottom end of the resistor 239 at A. C. ground potential and the upper end of said resistor at a voltage above ground determined by space current flow in the tube 277, not only with respect to the amplitude variations corresponding to the mechanical variations, but also with respect to the high frequency carrier wave variations. By this means when the switches 212 and 213 are in the position shown in drawing the modulation envelope together with its high frequency component may be delivered to the jack 28|.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention, as disclosed in the appended claims.

I claim:

1. Pick-up means of the character described for translating a mechanical variation into an electrical variation, including: a circuit comprising a rlrst impedance means and a second impedance means in series, said second impedance means being variable as a function of said mechanical variation; means coupled to said circuit for energizing said circuit with an alternating carrier voltage Wave; means connected to said circuit for eliminating at least a portion of said carrier wave, such means comprising at least one rectifying tube; means connected to said eliminating means for ultizing the ,Varying potential across one of said impedance means occasioned by the variation of said second impedance means, said last mentioned means comprising means selectively7 operable for removing the carrier component of the varying potential while retaining said varying potential I substantially unaltered; switching means movable between two positions, said switching means in one position providing the varying potential together With its carrier component, and in the other position providing the varying potential with the carrier removed; and compensating means connected to said switching means for providing that said varying potentials have the same amplitude when said switching means are in either position.

2. Pick-up means of the character described for translating a mechanical variation into an electrical variation, including: means to generate an alternating carrier voltage wave of substantially constant amplitude; means to modulate said carrier Wave in accordance with a function of said mechanical variation, said last named means comprising a single series circuit consisting of a single inductance member and a variable condenser in series, said condenser comprising a member adjustable with respect to a movable member whereby the capacitance therebetween can be set as desired, said movable member being adjacent said member and movable as a function of mechanical varlation whereby a variable capacitance exists between said members; means connecting said carrier wave generating means across said single series circuit only, a secondary inductance member inductively coupled to said single inductance member whereby the varying potential across said single inductance member occasioned by the varying capactive reactance of said condenser is induced in said secondary inductance member, means connected across the output of said secondary inductance member to eliminate those portions of the modulated carrier wave having an amplitude value less than a predetermined amplitude value, and means to adjust said predetermined amplitude value.

3. Pick-up means of the character described for translating a mechanical variation into an electrical variation, including: means to generate an alternating carrier voltage wave of substantially constant amplitude; means to modulate said carrier wave in accordance with a function of said mechanical variation, said last named means comprising a single series circuit consisting of a single inductance member and a variable condenser in series, said condenser comprising a member adjustable with respect to a movable member whereby the capacitance therebetween can be set as desired, said movable member being adjacent said member and movable as a function of said mechanical variation whereby a variable capacitance exists between said members, means connecting said carrier wave generating means across said single series circuit only; a secondary inductance member inductively coupled to said single inductance member whereby the varying potential across said single inductance member occasioned by the varying capacitive reactance of said condenser is induced in said secondary inductance member, rectifier means connected across the output ci said secondary inductance member to eliminate those portions of the modulated carrier waves having an amplitude value less than a predetermined amplitude value, and biasing means connected to said rectifier means.

DAVID L. ELAM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,718,494 Schurig June 25, 1929 1,939,067 Legg Dec. 12, 1933 1,943,986 Nyman Jan. 16, 1934 2,113,376 Janco Apr. 5, 1938 2,178,471 De Bruin Oct. 31, 1939 2,250,471 De Bruin July 29, 1941 2,266,608 Kuehni Dec. 16, 1941 2,288,838 Pike et al. July 7, 1942 2,349,992 Schrader May 30, 1944 2,355,088 Lavoie Aug. 8, 1944 2,361,634 Koch Oct. 31, 1944 2,410,295 Kuehni et al. Oct. 29, 1946 2,416,614 Crossley et al. Feb. 25, 1947 2,428,234 Mapp Sept. 30, 1947 2,433,378 Levy Dec. 30, 1947 2,439,047 Grinstead et al. Apr. 6, 1948 2,448,323 Boisblanc Aug. 31, 1948 OTHER REFERENCES Electronics, February 1946, Pressure-Time Curves by Brown, pp. 168 and 170. 

